Open any newspaper and you will likely see some mention of global warming—usually on the front page. The latest spate of Gulf Coast hurricanes has fueled the contentious debate that global warming is responsible for the increase in these storms. Conversely, support for the exploration of the Moon and Mars is fairly lukewarm. An October 2, 2005, Washington Post editorial urges terminating the new Vision for Space Exploration and using the funds for addressing such problems as recent hurricane damage. The same issue of that newspaper includes a survey on global warming: that shows that 56-percent of Americans are worried about global warming and want the government to do something about it.

We believe that over the next half-century we might be able to use lunar resources to construct a great shield at the Earth-Sun equilibrium point, or Lagrange L-1 point, about a million miles toward the Sun. Such a shield could block solar input exactly enough to counteract global-warming temperature increases. Moreover, the funding for this mammoth undertaking could be obtained from private sources.

Here's how. In our research, we have explored the technical feasibility of a giant parasol to counteract human-induced global warming. It would be 1,000 miles across, to reduce solar input by 0.1 to 0.2 percent, and would be built using lunar resources. It would not be a single structure, but a constellation of a very large cloud of small, free-flying parasols of gossamer-thin, lunar-made glass. The value of such an undertaking might be some trillions of dollars, just a few percent of the world GNP over five decades. It might, for example, be undertaken by private industry funded through carbon credits.

Although the scale of the project is gigantic, we can identify no showstoppers that would clearly make it financially or technically impossible. We urge that key technical issues be investigated now, so that the cost and feasibility of this option will become better understood over the next decade. Examples of key studies would be the manufacture of non-solarizing glass and structural alloys from lunar material, and the deployment, at L1 of a few (Earth-made) free-flyer parasol units to test station-keeping by solar sailing.

Global Warming

The Earth's surface temperature has risen by about 1 degree Fahrenheit in the past century, with accelerated warming during the past two decades. There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities. Increasing concentrations of greenhouse gases are likely to accelerate the rate of climate change. Scientists expect that the average global surface temperature could rise 1 to 4.5°F (0.6 to 2.5°C) in the next 50 years, and 2.2 to 10°F (1.4 to 5.8°C) in the next century, with significant regional variation. Global warming will have generally negative impacts on human life and the biosphere, so, to varying degrees, industry, scientists and policymakers are making significant efforts to mitigate the problem.

Most proposals for reversing global warming are aimed at lowering greenhouse gases, most notably the Kyoto Treaty, which aims to halt the rise—and eventually to lower—greenhouse gas emissions. Technical solutions to enable current levels of economic activity to proceed with lowered emissions are under investigation and development in private industry and at universities. These solutions focus on finding non-fossil fuels, and, more to the point, non-carbon-emitting energy sources. To this end, nuclear, solar and other energy sources are promising. Dave Criswell, a physics professor at the University of Houston, is exploring the possibility that solar energy captured on the Moon could be relayed to Earth to satisfy much of its future energy needs. But even if fossil-fuel burning were stopped tomorrow, the current exceptionally high level of carbon dioxide in the atmosphere would take more than a century to dissipate. Other solutions under study therefore include the capture and underground sequestration of atmospheric carbon.

Here we explore another approach for mitigating global warming, or indeed global climate change of any origin, by placing a shield at the Earth-Sun L1 point to redirect sunlight away from the Earth (or toward it to mitigate cooling).

Shields

Many experts have discussed a screen in space to mitigate global warming. A 2000 study by Bala Govindasamy and
Ken Caldeira showed that a screen yielding a 1.8 percent reduction in solar flux could fully reverse the current effect of the doubling of CO2. In a controlled orbit near L1, a screen would remain permanently lined up to block a small fraction of the solar radiation. To be effective, these huge "sunglasses" would have to be 1,000 miles across, and even at gossamer thickness would weigh millions of tons.

In 1989, engineer James Early, whose work fostered the creation of Telstar-1, the first American communications satellite, proposed a blocker made of thin ribbed glass to deflect the sunlight. He recognized that the costs of launching so much mass from Earth could be prohibitive, and that a practical solution might be found by making the shield from lunar material. Solar power could be used to process the material into glass and structural elements, and to drive a magnetic rail for launch into the L1 orbit.

Early's idea is now worth revisiting. The value of maintaining a viable climate can be determined in different ways, and is likely to be in the range of $5 to $10 trillion—again, just a few percent of world GNP over the next 50 years. In order to find this balance, research is needed now to better understand if a shade could be implemented within the above cost ceiling, and within a few decades.

To steer the full spectrum of sunlight away from the Earth, the glass needs an average thickness of about 2 micron—a fiftieth that of a human hair. Even at such light weight, a thousand- mile diameter sheet will weigh 10 million tons. To build the shield in 30 years, glass production would need to be about 1,000 tons a day, along with several hundred tons a day of titanium or aluminum for structural components. The electric power needed to mine the ore and to process it, and to accelerate 1,500 tons a day to escape the Moon and reach the L-1 point, at a 3 km/sec launch speed would be about 500 megawatts. This would require a solar plant with a couple of square kilometers of solar cells weighing about 2,000 tons.

The shade would be built not as a single structure but as a constellation of many identically sized, free-flying parasol elements. For example, if each self-contained unit were as small as a 14-meter square and weighed about 1 kilogram, ten billion units would be needed to make up the shield. In manufacture, the Moon-derived structural metal would be fashioned into ultra-lightweight support struts at free-orbiting factories near L1. The screen itself, cut in squares from a 14meter-wide roll of thin glass also delivered from the Moon, would be attached to a structural cross with four 10-meter-long struts connected at a center hub. Each unit would include tilting reflecting panels, to be used as solar sails for initial placement within the constellation and for station-keeping, particularly to stabilize any drift in the unstable longitudinal direction.

We envision the constellation as being like a large shoal of fish or flock of birds, with station-keeping control largely by autonomous computers in each unit to prevent collisions or self- shadowing. A local positioning system like GPS would also be used.

To make ten billion units of 14-meter squares in 30 years (10,000 days) would require manufacture and placement of a million units a day at L1. If there were 1,000 factories working in parallel, each factory would have to complete a unit in little more than a minute. The factories would need to use sophisticated robots made on Earth, and might weigh in the range of 1 to 10 tons each.

Economics

We can make some estimate of the value of global warming from the current "carbon credit" market. Following the 1997 Kyoto Treaty, individuals or nations can purchase excess "credits" for atmospheric emission of carbon dioxide from nations that produce less than their allocated treaty quota. This amount varies between a few dollars to more than $60 per metric ton. The doubling of carbon dioxide in the Earth's atmosphere that the shield described above would alleviate corresponds to about 400 billion tons. Mitigating this using the carbon credit analogy would be worth trillions of dollars. The cost might be financed by selling shield credits to both nations and industries. If a group were to purchase a set amount of shield structure, this would translate directly into carbon credits. In this way, the entire project might be financed "off budget" from government funds.

How to Proceed

The shield would require three major high-tech elements that would likely be manufactured and launched from the Earth. The first would be the package to enable material production and launch on the Moon. This would include the robots, electronics, solar cells, wire, bearings, motors and high-temperature ceramics for the lunar manufacturing and for the rail gun to launch the manufactured items back off the Moon. It would also include the pilot facilities on the Moon to bootstrap the local manufacture of structural elements used in full-scale lunar operations.

We estimate the total mass to be delivered to the Moon at around 10,000 tons. At L1, the 10 billion control units at 1 gram will also each weigh 10,000 tons, and so will the 1,000 robotic assembly factories if we allow 10 tons each.

The total mass to be launched from Earth for the entire screen project of 30,000 tons is less than 0.2 percent of the screen's final mass, and even at today's high launch costs of $20,000/kg would cost less than $1 trillion to launch. Reductions in launch cost, however, would be desirable to give cushion and flexibility to the project. The cost of manufacturing the elements to be launched, including the development of the manufacturing and robotic techniques, might bring their costs to $10,000/kg or $3 trillion. Another $20 billion per year might be allocated for project management. The estimated total of less than $5 trillion is not out of line with the value of the shield—$5 to $10 trillion over several decades.

The developments needed for this application with potentially immense benefits to human life on Earth are consistent with the New Vision for Space Exploration, which aims at more affordable access to space beyond near-Earth orbit. We identify several specific near-term activities that should be undertaken. It would be desirable and practical to develop and place a few prototype blocker units at L1 within a few years, to test positioning and station keeping by solar sails. The materials would be consistent with expected lunar products, and the units should have the correct mass, about 1 kg for the example we have chosen.

A key requirement for the glass is that it remain crystal-clear for a century. The Sun produces darkening or "solarization" in some glass materials over long periods of time. We need to find glass that is resistant to this effect. Prospecting for the optimum lunar ores will be required. Techniques to produce the glass ingots on the Moon and to mass-produce the ribbed sheets need to be developed and tested. We envision that ultimately the glass would be rolled up for launch.

Another valuable near-term step is, thus, to computer-simulate and optimize the "collective intelligence" of the blocker swarm for robustness and stability. The free-flyer control units will have to last for a century or more. Since there will likely be millions of failures, there must also be a system to identify failed units and sweep them out for refurbishment or replacement before the swarm is damaged.

In Conclusion

A global-warming Sun shield is a very challenging project, to say the least, but is not clearly impossible within the financial target. It seems certain that it would attract the best and brightest from across the world to solve the myriad of challenges involved, in a way that has not happened since Apollo or the Manhattan Project. It might also represent the first truly large-scale commercial and private-sector use of space, and would certainly be of benefit to the entire population of Earth. Now is the time to begin in earnest the development and testing of these critical technical steps.

Dr. Roger Angel is a Regents Professor at the University of Arizona and is on the faculty of the UA astronomy department and the Optical Sciences College. He is the director of the renowned Steward Observatory Mirror Laboratory and the Center for Astronomical Adaptive Optics and the 2006 recipient of the Joseph Weber Award for Astronomical Instrumentation presented by the American Astronomical Society.

Dr. Simon "Pete" Worden was appointed as a Research Professor of Astronomy at the University of Arizona on May 1, 2004 and a Research Professor of Optical Sciences and Planetary Sciences in March 2005. He retired as a Brigadier General from the United States Air Force in March 2004. Subsequent to the publishing of this article, he became Director of NASA Ames Research Center.